I am replacing this window and am trying to figure out where I should be measuring from (to know what a suitable replacement for it should be):

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  • For the height/vertical measurement, should I be measuring from point A to H, or from D to J, or between some other two places?
  • For the width/horizontal measurement, should I be measuring from B to the same board on the other side of the window, or between some other two places?

Thanks in advance!

1 Answer 1


Depends on whether you're redoing the jambs and trim. Many insta-window replacement companies reuse jambs so they don't need to get into casing, extension jambs, and exterior trim at all. It allows for a quick, relatively inexpensive job, but smaller windows, as around an inch of space is lost on all sides. It also leaves any existing energy efficiency issues in place, such as air leaks and insulation deficiencies.

So, you decide. If you're willing to rework the siding trim and would prefer to modernize energy efficiency, you'd measure to the framing. If you're keeping the original jambs, measure those. It's also possible that you'd decide on some combination of those based on your trim details.

Sometimes you have to take things apart to come to a conclusion.

Related resource: Window anatomy, by example

  • Thanks @isherwood (+1) -- jambs, framing, these are terms I am currently working on learning. I did ask this question on Window anatomy, so once I understand the terms over there, I'm sure this answer will make a lot more sense! Commented Sep 27, 2022 at 13:33
  • I think (keep me honest) it makes sense to remove the entire "unit" which I guess I'm calling "all the parts that can be removed without affecting the framing". So: jambs, trim and sashes. I the jambs are still in decent shape, it probably makes sense to keep them. Unless keeping them changes the dimensions of the sashes such that we would need to order "custom sized windows" from our local Big Box Retailer; custom sized means they're about twice as expensive...so my strategy is also largely driven by price. Commented Sep 27, 2022 at 14:36
  • So I think I'll keep the jambs if they are good and they don't force us to go with a "custom sized" order, and I'll replace them otherwise. Does this sound like a valid strategy to you, or am I way off base? Commented Sep 27, 2022 at 14:36
  • It's entirely up to you. Personally, since you have wood siding (which is easily modified), I'd be starting fresh at framing so I have the largest windows possible and the least old stuff left behind. You do you, though.
    – isherwood
    Commented Sep 27, 2022 at 14:38

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